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Introduction Due to a rapid increase in the number of technologies and devices operating in the license- free 2.4 GHz band – Radio interference becomes an increasing problem for low-power wireless sensor networks It has been shown that interference from other devices reduces sensor network performance – as it causes packet loss, reduces throughput, increases delay, and drains the sensor nodes’ limited energy reserves

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SoNIC Sensor Network Interference Classification (SoNIC) system Takes a novel path to interference detection Rather than actively sampling the spectrum A node using SoNIC detects interferers by – considering individual corrupted 802.15.4 packets – packets that the node has received, but for which the received payload did not match the packet’s checksum

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Fingerprint Through extensive measurements, it has been established that different interferers corrupt individual 802.15.4 packets in distinct patterns – thereby leaving a “fingerprint” on the packet The interferer’s fingerprint becomes visible in – (i) how the signal strength varies during packet reception – (ii) in the link quality indication (LQI) associated with the packet – (iii) which bytes of the payload are corrupted

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Memory Overhead SoNIC’s memory requirements are dominated by the need to store the decision tree in the sensor node’s RAM, which requires 1.8 KB SoNIC uses 1 KB to store corrupted packets in the FIFO buffer for later matching Furthermore, another static buffer of 128 bytes is used to store valid packets – so they can be matched after they have been processed by the network stack

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Computational Overhead comprised of feature calculation and classification Select 1000 packets at random from the testing set Measure the time it takes to calculate features and classify them on a TelosB node

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Computational Overhead Mean feature calculation time of 26.5 ms (σ = 7.0 ms) is dominated by normalizing the RSSI values – accounts for about 60% of the total calculation time – because it requires repeated 32-bit integer divisions One classification takes 1.2 ms on average (σ = 0.5 ms)

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Mitigation The mobile sink implements two exemplary mitigation strategies When WiFi interference is detected, the mobile sink switches communication to another 802.15.4 channel, separated 30 MHz from the interfered channel In this way, it avoids a frequency overlap with the WiFi channel

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Mitigation To mitigate microwave interference, the nodes time their transmission – so they do not coincide with the microwave emissions Microwave emissions are very regular in time, following a 10 ms on, 10 ms off pattern

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Discussion SoNIC’s classifier distinguishes between – WiFi – Bluetooth – microwave oven interference – packets that are corrupted due to low TX power To add detection capabilities for a new interference type – suitable features must be defined – classifier needs to be retrained

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Discussion Have not performed any explicit experiments with multiple interferers SoNIC is currently designed to identify the main interferer The voter chooses the most common class of packets in the window as the interfering state and passes this state to the application To address multiple interferers of different kinds, – Should change the voting algorithm to, for example, estimate the likelihood of the presence of a specific interferer

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Sensor networks that use 802.15.4 at 2.4 GHz face cross-technology interference from many other technologies operating in the same frequency band Previous research has shown that interference mitigation in sensor networks can be more effective if the type of interference is known This paper addressed the problem of classifying and detecting interference in a sensor network

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Conclusion Introduced a novel approach to interference classification that considers individual, corrupted 802.15.4 packets – rather than using costly continuous spectrum sampling Evaluation has shown that our implementation of the approach is sufficiently lightweight for use on resource-constrained sensor nodes It correctly detects the predominant interferer in an uncontrolled office environment